1. Field of the Invention
The present invention relates to a track-offset compensation technique in optical disk drives. More particularly, the invention relates to a method and a system for compensating the track offset in optical disk drives, which is applicable to high-density optical disks with wobbled grooves (i.e., optical disks using the groove wobble recording), and an optical disk drive using the method or system.
2. Description of the Related Art
Conventionally, optical disk drives utilize servo mechanism to control the laser beam irradiated from the optical head to an optical disk loaded in the focusing direction and the radial direction, thereby stabilizing the recording and reproducing (i.e., writing and reading) operations. Typically, the servo-control in the focusing direction is termed “focusing servo-control” while the servo-control the radial direction is termed “tracking servo-control”.
With the “focusing servo-control” or “tracking servo-control” in an optical disk drive, an error signal is always detected during the operation of the drive. If the error signal is detected, the optical head is subjected to specific feedback control in such a way that the error signal thus detected has a specific value (typically, 0) Thus, the spot of the irradiated laser beam is controlled to satisfy the desired focusing and tracking conditions to thereby move along a desired track of the disk.
However, the error signal includes various offset factors. Therefore, even if the electrical error signal has a value of 0, the optical spot does not always scan through a desired point on the disk. As a result, it is necessary to eliminate all the offset factors or to implement proper feed-back control of the spot while taking these factors into consideration. In this case, the feed-back control needs to be performed in such a way that the error signal has a specific value not equal to 0. This control may be termed the “offset compensation” below.
The offset factors include various types of errors. Examples are positional deviation of the detector in the optical head; bias of the optical beam intensity; inclination between the optical head and the optical disk; surface unevenness of the optical disk; and offset in the electrical system. Thus, it is difficult to eliminate or remove all of these offset factors and to implement necessary servo-control mechanisms while considering all the factors.
Recently, several standards have been defined for Digital Versatile Disks (DVDs), such as DVD-R (Recordable), DVD-R/W (Recordable/Writable), DVD+RW (ReWritable), and DVD-RAM (Random-Access Memory), in the optical disk industry. These standards have been considered as particularly promising standards for next-generation, high-density optical disks and thus, vigorous developments have been being made on these types of optical disks.
Optical disks according to the above-described DVD standards have guiding grooves wobbled at a specific period. If the reflected light beam by the disk is reproduced using the so-called push-pull method, a “wobbling signal” having the same period as the wobbling period of the grooves is obtainable. Thus, the wobbling signal is used for example, to achieve the rotational synchronization of the spindle for rotating the disk and/or to generate the write clock for data writing operation. Considering this fact, a serious problem will occur if the wobbling signal is not well reproduced as desired.
In particular, for the optical disks according to the DVD+RW standard, the insufficient or inadequate reproduction of the wobbling signal leads to a critical problem. This is because part of the wobbling signal is inverted in phase in the disk of the DVD+RW type and at the same, time, specific address information or the like is buried in the positions thus phase-inverted. This method or disk structure is termed “ADIP”.
It is known that the quality of the wobbling signal degrades largely after data is recorded or stored on the disk. This is because light and shade spots (or, marks and spaces) are created by the stored data. This badly affects the quality of the wobbling signal. Therefore, it is necessary for the wobbling signal to be synchronized with the period of the wobbled grooves even after data is stored on the disk. When the disks are of the DVD+RW type, a critical problem may occur if the wobbling signal is unable to be reproduced well. This is because the buried address information cannot be read out, in other words, the ADIP signal is not reproduced well.
It is known that the wobbling signal and the ADIP signal are highly sensitive to track offset. According to the inventor's test on the optical disk equivalent to the DVD+RW type, the relationship of the track offset with the synchronization of the wobbling signal with the wobbling period of the grooves and the block error rate (BLER) (where one block is equal to one ADIP word) of the ADIP signal was as follows:
When the track offset was set at the optimum position, the wobbling signal was synchronized with the wobbling period of the grooves and at the same time, the BLER of the ADIP signal was limited to approximately 60%. However, when the track offset was shifted from the optimum position by only approximately 0.02 μm, the synchronization of the wobbling signal was unable to be achieved and the BLER of the ADIP signal was raised to approximately 80%. At this time, the signal-to-noise ratio (SNR) of the wobbling signal was measured in a manner according to the EGMA standards. In this case, the SNR thus measured was approximately 38 dB not only when the track offset was set at the optimum position but also when the track offset was shifted from the optimum position by approximately 0.02 μm. This means that the value of the SNR changed scarcely even if the track offset varies within this range. In other words, such a minute change of the track offset greatly affects the synchronization of the wobbling signal and the BLER of the ADIP signal. As a result, the track offset needs to be well controlled with very high accuracy (i.e., the track offset needs to be optimized) even if no change is observed in the SNR of the wobbling signal due to minute deviation of the track offset.
One of the prior-art methods for eliminating or compensating the track offset is disclosed in the Japanese Non-Examined Patent Publication No. 2000-163765 published in 2000. In this method, wobbling pits for detecting the track offset are formed on the disk and then, the track offset is eliminated by using the signal generated by the wobbling pits.
Another prior-art method for compensating the track offset is disclosed in the Japanese Non-Examined Patent Publication No. 9-259455 published in 1997. In this method, the fact that the amplitude of the wobbling signal varies with the motion of the lens is utilized. The shift of the lens compensates for the track offset.
Further prior-art methods for eliminating or compensating the track offset are disclosed in the Japanese Patent. No. 2606509 issued on Feb. 13, 1997 (which corresponds to the Japanese Non-Examined Patent Publication No. 5-151600) and the Japanese Non-Examined Patent Publication No. 9-7200 published in 1997. In these two methods, the relationship of the track offset with the error rate of the wobbling signal or the ATIP (Absolute Time In Pregroove) signal is examined. Then, the track offset is adjusted at a position where the error rate thus examined is minimized.
With the above-described prior-art method for eliminating the track offset with the wobbling pits, which is disclosed in the Publication No. 2000-163765, it is necessary to form the wobbling pits, which decreases the formatting efficiency and the storing capacity of the disk. Moreover, the formation of the wobbling pits is not adopted in the current DVD standards and thus, this method does not have desired versatility.
With the above-described prior-art method for compensating the track offset with the lens shift, which is disclosed in the Publication No. 9-259455, offset factors other than the lens shift are unable to be considered. Thus, the track offset is not completely corrected.
With the above-described prior-art methods for eliminating the track offset with the wobbling pits, which are disclosed in the Patent No. 2606509 and the Publication No. 9-7200, the error rate is actually measured. Therefore, the stored signal on the disk needs to be sampled at lease 106 times if the error rate is approximately 10−5. Thus, it takes a very long time for track-offset compensation.